In high-frequency surgery, in the following also referred to as HF surgery, an alternating current having a high frequency is guided through the human body or a certain tissue area of the human body in order to specifically damage or cut the tissue at a defined place. A substantial advantage over a conventional cutting technology using a scalpel is that the bleeding can be arrested by closing the affected vessels during the cutting process. During resecting tissue, for example, such as in the event of a partial lung resection or hepatic lobe resection, such an intervention is therefore not carried out as usual by means of a clip suture device; rather, the tissue is “welded” or “sealed” by means of the HF current of such a TFT instrument. The advantage of this method is to be seen clearly in the fact that there are no implants (e.g. metal clips) which remain in the body.
Basically, HF electro-surgery can be divided into two application techniques, which are the monopolar application technique and the bipolar application technique.
The monopolar technology is still the most common technique in use. Here, one pole of the HF voltage source is connected to the patient via a surface area which is as large as possible. This electrode is referred to as the neutral electrode. The other pole is the actual surgical instrument (active electrode) which is manually guided by the surgeon. The current flows through the path with the smallest resistance from the active electrode to the neutral electrode. In the direct vicinity of the active electrode, the current density is highest, which is the place where the strongest thermal effect occurs.
The neutral electrode is shaped with a surface area as large as possible, so that the current density in the body is kept small and no burns will occur. Due to the large surface area, the skin on the neutral electrode is not noticeably heated.
With the bipolar technology and in contrast to the monopolar technology, the current flows only through a small part of the body—in fact through that part where the surgical impact (cut or coagulation) is desired. Two electrodes which are isolated with respect to each other and between which the HF voltage is applied, are directly brought to the site of operation. The electrical circuit is closed via the tissue situated in between. The thermal effect will then occur in the tissue between the electrodes.
As compared to the monopolar technology, 20-30% less power is required. The surrounding tissue will not be damaged as there is no current flow, and any measuring equipment on the patient (e.g. ECG) will not be disturbed. Therefore, this procedure is particularly well suited for critical and precise applications such as in micro, neuro and ENT surgery.
In order to avoid a failure of the sealed or welded portion of the tissue, especially in the bipolar application technique, the parameters acting on the tissue have to be detected and adjusted on the instrument as precisely as possible. In order to ensure this, an exact monitoring of the following parameters is indispensable:                temperature on/in the tissue between the electrodes,        clamping pressure exerted on the tissue by the electrodes,        tissue impedance,        mutual electrode distance, and possibly        position of the instrument or position of the electrodes relative to each other.        
For those frequencies which are used in HF surgery, the body tissue behaves like an ohmic resistance. Here, the specific resistance highly depends on the type of the tissue, with the power input for the same temperature increase of the tissue being directly proportional to the specific resistance. The specific resistance of muscle tissue and highly perfused tissue is relatively low. The specific resistance of fat is higher by about factor 15 and the one of bones by factor 1000. The effective resistance, however, also depends on the type and the shape of the electrodes as well as on the intended degree of destruction of the tissue. Thus, the form and the level of the current have to be exactly adapted to the type of the tissue where the operation is carried out as well as to the employed electrodes and should yield a constant and uniform result throughout the entire electrode length.
The quick and efficient arrest of bleeding with coagulation application is used if there is no spontaneous coagulation and replaces—for small vessels—in most cases the costly fibrin sealants or the expensive ligature. The term “coagulation” includes two different surgical techniques here: The in-depth coagulation and the (electrical) arrest of bleeding.
In the in-depth coagulation, the tissue is heated up in large areas to 50-80° C. This is carried out with ball, plate or roller type electrodes and serves for the subsequent ablation of the tissue. In this process, a high current density and a current without impulse modulation are used. The coagulation depth can be influenced by the magnitude of the amperage.
For stopping bleeding, an impulse-modulated HF current is used on clamps and forceps. The blood vessels are gripped with the tips of the tools and constricted through dehydration until they are completely closed. This is carried out in the bipolar mode, in rare cases monopolar forceps are used.
The process of cutting the tissue (instead of cutting with the scalpel) is referred to as electrotomy in HF surgery. During the cutting process, the HF surgical device (TFT instrument) is operated in the monopolar mode with a needle or narrow lamella (blade). Recently, also bipolar scissors are applied with great success for cutting.
Related state of the art is known from the documents DE 10 2008 008 309 A1, DE 10 2010 031 569 A1 and US 2008/0 183 251 A1. The reference DE 10 2010 031 569 A1 discloses an electrosurgical instrument for cutting and sealing tissue, the instrument comprising:
a) a first and a second cheek element in opposing relationship relative to each other, the first cheek element comprising an inner surface adapted to cooperate with the inner surface of the second cheek element in order to grip tissue therebetween; at least one of the cheek elements is movable with respect to the other one, so that the cheek elements are selectively operable between an open position in which the cheek elements are arranged in a spaced relationship relative to each other, and a closed position in which the inner surfaces of the cheek elements work together to grip the tissue between them;
b) means for causing a movement of the one or each cheek element, in order to operate the cheek elements between the open and closed positions;
c) a first electrode as the coagulation electrode on the inner surface of the one of the cheek elements;
d) a second electrode as the coagulation electrode on the inner surface of the one of the cheek elements;
e) an isolation element separating the first and second electrodes, the first and second electrodes being connectable to opposite poles of an electrosurgical generator;
f) a third electrode as the cutting electrode on the inner surface of the first cheek element, the third electrode being connectable to a pole of the electrosurgical generator; and
g) a fourth electrode on an external surface of the first cheek element separate from the inner surface, the fourth electrode being connectable to a pole of the electro-surgical generator;
the electrosurgical instrument being capable of selectively causing a coagulation of the tissue between the first and second electrodes and/or a process of cutting the tissue touched by the third electrode and/or a treatment of the tissue touched by the fourth electrode.
With the surgical HF instruments (TFT instruments) of the bipolar type, however, preferably consisting of two electrode legs which can pivot in the manner of scissors or pincers, there is the basic problem that the opposing HF electrodes (sealing/welding electrodes) are not in an exactly parallel orientation upon closing the jaw part comprised of the electrode legs, so that the clamping pressure exerted on the tissue clamped therebetween is not homogenous along the electrodes. As the clamping pressure is one of those above-mentioned parameters having an evidently large influence on the treatment outcome, an inhomogenous clamping pressure has a negative impact on the seam quality along the two electrodes. In addition, an irregular gap width implies an irregular flow of current over the electrode length, which is also disadvantageous for the treatment outcome (coagulation quality). Here, the influences of the mentioned parameters on the treatment outcome are so strong that already small deviations from the optimum values can have significant effects.